1. Field
Example embodiments of inventive concepts relate to a storage device and a method for reading the same.
2. Description of the Related Art
Semiconductor memory devices are generally classified into volatile semiconductor memory devices and non-volatile semiconductor memory devices. Although read and writing speeds of the volatile semiconductor memory devices are relatively fast, data stored in the volatile semiconductor memory devices is erased when an external power supply source is shut off. On the other hand, the nonvolatile semiconductor memory devices retain stored data even when an external power supply source is shut off.
In MROMs, PROMs and EPROMs, generally, users have difficulty in updating stored data because it is inconvenient to autonomously erase and write data in a system. On the other hand, since it is possible to electrically erase and write data from/to EEPROMs, EEPROM are being increasingly applied to system programming or auxiliary memory devices that require constant updates. Particularly, since having a higher degree of integration than typical EEPROMs, flash EEPROMs are very useful for large-capacity auxiliary memory devices. Among the flash EEPROMs, a NAND-type flash EEPROM (hereinafter, referred to as a NAND-type flash memory) tends to have a higher degree of integration than other flash EEPROMs.
Recently, as high-integration requirements for memory devices increase, multi-bit memory devices that store multiple bits in one memory cell are being widely used. In the memory cells of a multi-bit flash memory device, the intervals between threshold voltage distributions must be generally close. Thus, the number of program/delete cycles (or durability) that does not cause reduction of data retention characteristics and quality is an issue in regard to data reliability.
Reduction of the sizes of a semiconductor memory device and operation voltage results in an increase of data reading errors with respect to noises. Noise sources include thermal noises and Random Telegraph Signal (RTS) noises.